Use createCommitOnBranch for file write tools

[bongsu-rapportlabs]

Summary

• Use GraphQL createCommitOnBranch for create_or_update_file and push_files commits.
• Keep existing SHA validation and tool inputs, and preserve the create_or_update_file response by fetching the updated file metadata after the commit.
• Remove the now-unused REST contents response converter.

Closes #2771.

Testing

• mise exec go@1.25.0 -- ./script/lint
• mise exec go@1.25.0 -- go test ./... -count=1